The objective of this proposal is to bring to bear state of the art theoretical methods to the study of the mechanisms of ribozyme catalysis and the factors that regulate reactivity. An overarching theme in the proposal is to facilitate active collaborations with a network of experimental groups in order to progress toward a consensus view of mechanism that may, ultimately, contribute to a deeper understanding of more complex cellular catalytic RNA systems such as the ribosome and spliceosome. The proposal focuses on application to a series of archetype ribozymes that catalyze site-specific phosphodiester bond cleavage (and ligation) but that have different active site architectures and catalytic requirements. As a baseline, the same reaction will be studied in a non-enzymatic dinucleotide model in solution, and catalyzed by the protein enzyme analog, RNase A. The tandem study of alternate mechanistic strategies and the factors that govern reactivity will provide penetrating insight into the rational design of new biomedical technology. However, these applications demand new methodological advances. Of key importance are accurate and efficient quantum mechanical/molecular mechanical (QM/MM) methods for catalysis, reliable molecular simulation force fields for RNA and metal ions, and efficient methods for sampling free energy surfaces and conformational transitions. Toward this end, we propose to: 1) improve the QM, MM and QM/MM models for ribozyme catalysis, 2) develop a new QM/MM method for prediction of pKa shifts in ribozymes; 3) develop a novel free energy expansion approach to extend our QM/MM methods to the ab initio level; 4) develop efficient path methods to study chemical mechanisms and conformational transitions in ribozymes. In this way we hope to greatly extend the predictive capability and range of application of state of the art theoretical methods to RNA catalysis. PUBLIC HEALTH RELEVANCE: The goal of this proposal is to use quantum mechanical and molecular simulations methods to study the mechanisms whereby molecules of RNA catalyze important chemical reactions. The insight gained by these studies will enhance our understanding of the fundamental role RNA plays in cells and facilitate the design of new RNA-based biomedical technology.